1. Field of the Invention
This invention relates to a low cost polyurethane elastomer having high resilience and clarity that can be processed safely.
2. Description of Related Art
Polyurethane elastomers are well known; see, e.g., U.S. Pat. Nos. 4,294,951; 4,555,562; and 5,599,874. Polyurethane elastomers can be formed by reacting an diisocyanate, e.g., diphenyl methane diisocyanate (MDI), toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), and the like., with an organic polyol, e.g., polytetramethylene ether glycol (PTMEG), polyester or polycaprolactone glycol (PE), homopolymers and copolymers of ethylene oxide and propylene oxide (E/PO), and the like, and a chain extender, e.g., an aliphatic diol, such as, 1,4 butanediol (BD), or an aromatic diamine, such as, diethyltoluene diamine (DETDA). Catalysts, such as, triethylene diamine (TEDA), can be used to increase the reactivity of the components. Additional components, such as, UV stabilizers, antioxidants, dyes, antistatic agents, and the like, can be added, if desired.
In preparing the polyurethane prepolymer, the organic diisocyanate monomer is employed in a stoichiometric excess of the diisocyanate monomer in relation to the polyol (an NCO:OH ratio greater than 1:1, usually about 2:1 or greater). The use of such an excess of diisocyanate monomer, however, results in an undesirable amount of unreacted volatile diisocyanate monomer in the prepolymer reaction product mixture.
Several techniques have been described in the art as useful for reducing the amount of diisocyanate monomer in the prepolymer reaction product mixture. For example, British Patent 1,101,410 and U.S. Pat. No. 4,182,825 describe a process for distilling the prepolymer reaction product under vacuum conditions to reduce the amount of diisocyanate. U.S. Pat. No. 4,061,662 describes a process for the removal of unreacted toluene diisocyanate from prepolymers by passing the prepolymer reaction product through a column containing a molecular sieve. U.S. Pat. No. 4,385,171 describes a method for the removal of unreacted diisocyanate from polyurethane prepolymers by codistilling the prepolymer reaction product with a compound that boils at a temperature greater than the boiling point of the diisocyanate. U.S. Pat. No. 4,888,442 describes a two-step process consisting of a first step of distilling the prepolymer reaction product to remove the bulk of the diisocyanate and then, in the second step, adding a solvent to the distilled prepolymer reaction product and passing the resultant mixture through an agitated thin-film evaporator. According to this patent, the vaporization temperature of the solvent should be exceeded under the conditions employed in the thin layer evaporator. U.S. Pat. No. 4,288,577 describes the removal of unreacted methylene bis(4-phenyl isocyanate) (MDI) via solvent extraction with hexane.
The procedures described above relate to reduction of such diisocyanate compounds as toluene diisocyanate (TDI), methylene-bis-(4-phenyl)isocyanate (MDI), and the like. Other diisocyanate monomers with high melting points, such as para-phenylene diisocyanate (PPDI), can be removed by the process of U.S. Pat. No. 5,703,193, which comprises distilling the polyurethane prepolymer reaction product mixture in the presence of a combination of at least one inert first solvent with a boiling point below the boiling point of the residual organic diisocyanate monomer and at least one inert second solvent with a boiling point above the boiling point of the the residual organic diisocyanate monomer, at a temperature that exceeds the vaporization temperature of the residual organic diisocyanate monomer and that is below the decomposition of the polyurethane prepolymer.
In certain applications, such as, in-line skate wheels, skateboard wheels, and swim fins, clarity of the elastomer is appealing to consumers. Indeed, the vast majority of in-line skate wheels sold today are clear. A clear elastomer is one through which standard 12-pitch type can easily be read, even where the elastomer is two inches thick.
Performance aspects are also important. Resilience is a key performance characteristic, since it indicates the amount of energy lost when the elastomer is flexed, and is, thus, a measurement of the rolling resistance contributed by the elastomer to a wheel. Wheels with lower rolling resistance require less energy for motion at a given speed, which is highly desirable. Resilience can be measured by determining the percentage of the original height to which a 1/2" steel ball will rebound after being dropped onto a 1/2" thick elastomer sample from a height of one meter. In this measurement, it is important that the elastomer be mounted in such a manner as to prevent it from moving at impact.
Currently, state of the art skate wheels are composed of MDI, PTMEG, and one or more diol chain extenders. When processed properly, this composition can provide a material having high resilience (70+%) and good clarity, but the cost of such a system is relatively high because the major ingredient is PTMEG, which is the most expensive of the three main classes of polyols. It would be desirable to use a lower cost polyol; however, lower cost polyols, such as, PE and E/PO, provide a product having a resilience of only about 50% of the maximum, when processed in a manner similar to that used with PTMEG. Nonetheless, these polyols have been used in lower cost skates, where price begins to outweigh performance in importance.
MDI is used because it is an inexpensive isocyanate, provides good resilience, and can be processed at low cost. Low cost processing is possible because of the low viscosity of MDI systems, which permits the rapid filling of molds and provides fast gel times without entrapping bubbles. This low viscosity is possible because MDI has a very low vapor pressure, whereby systems can contain large amounts of free MDI monomer without presenting an airborne safety hazard. Other isocyanates, such as, TDI and IPDI, are best handled as "full prepolymers", which are almost completely pre-reacted with polyol, because their higher vapor pressures make safe handling with high isocyanate monomer contents difficult or impossible.
It would be highly desirable if a method for using the low cost PE or E/PO polyols could be found that would produce a product wherein resilience, clarity, cost, and safety of processing are not sacrificed.